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CO Oxidation on Rutile-Supported Au Nanoparticles.

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Zuschriften
Heterogeneous Catalysis
The relaxed structure of the [Au10] cluster supported on
rutile TiO2(110) is shown in Figure 1. CO binds above the
highest Au atom (labeled 1 in Figure 1) with an adsorption
energy of 0.95 eV. Adsorption is significantly more exo-
CO Oxidation on Rutile-Supported Au
Nanoparticles**
Ioannis N. Remediakis,* Nuria Lopez, and
Jens K. Nørskov
Gold is usually catalytically inert in chemical processes.[1] On
the other hand, nanometric gold particles supported on oxides
have been found to be catalytically active, even at low
temperatures.[2, 3] A number of effects may contribute to the
enhanced reactivity of small gold clusters: an odd or even
number of electrons, a metal–insulator transition below a
certain cluster size,[3, 4] charge transfer from the support,[5, 6]
strain,[7] or the presence of undercoordinated step and corner
atoms.[4, 7–11] The catalytic properties of gold are also reported
to be affected by the supporting oxide and the method of
preparation, which suggests that atoms near the boundary
between the gold cluster and the oxide may have a key role in
the oxidation of CO.[12, 13] In addition, defects in the support
can stabilize the metal particles and may change their
catalytic properties.[10]
In a pioneering work, Molina et al. calculated the
energetics of CO oxidation on an infinite array of Au rods
supported on TiO2.[14] They found that in the minimumenergy oxidation path, CO and O2 are adsorbed near the gold/
support interface. Similar results, for a two-layer gold strip
were found by Liu et al.[15] Herein, we take one further step
and present the first theoretical study of CO oxidation on a
finite gold nanoparticle supported on rutile (TiO2). On this
basis, we propose an additional mechanism for the oxidation
of CO that takes place on corner Au atoms and does not
directly involve the gold/support interface. The [Au10] cluster
that we have studied has a size of approximately 0.7 nm,
which is at the lower end of the size-range for active
nanoparticles reported in experiments.[3] This model system
has most of the important features that have been suggested
to contribute to the catalytic activity of gold: it has a 3D
structure with two layers, it contains the characteristic (111)
and (100) facets of gold clusters, and, most importantly, it
takes into account both the finite size of the cluster and the
redox character of the support.
[*] Dr. I. N. Remediakis, Prof. Dr. J. K. Nørskov
Center for Atomic-Scale Materials Physics
Department of Physics
Technical University of Denmark
2800 Lyngby (Denmark)
Fax: (+ 45) 4593-2399
E-mail: remed@fysik.dtu.dk
Dr. N. Lopez
Departament de Quimica Fisica
Universitat de Barcelona
C/Marti i Franques 1, 08028 Barcelona (Spain)
[**] We acknowledge support from the Danish Center for Scientific
Computing through grant no. HDW-1101-05. N.L. acknowledges
MCyT for financing her work through the RyC program.
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 1. Relaxed geometry for an [Au10] cluster supported on reduced
rutile TiO2(110). Au, Ti, and O atoms are represented by yellow, gray,
and red spheres, respectively. The structure is periodic in both lateral
dimensions.
thermic compared with that calculated for a free, disk-shaped
gold cluster (0.6 eV).[8] This difference is mainly due to the
decrease in the coordination number of the Au atom in the
supported cluster.[10] Recently, Meier and Goodman[16]
reported a binding energy of 0.8 eV for CO on TiO2supported Au bilayer clusters and 0.5 eV for CO on unsupported Au clusters. Within the error range associated with
experimental and calculated values, these results are essentially the same as those calculated. For co-adsorbed CO and
O2, the minimum-energy structure (1.59 eV, relative to gasphase CO and O2) involves CO adsorbed on top of Au(1)
(Figure 1) and O2 bonded between Au(5) and Ti(7). This
structure is 0.4 eV lower in energy than the best co-adsorption
geometry for CO and O2 that involves Au atoms only (see
below).
The ensemble, which gives rise to such strong bonding for
O2, is a corner gold atom in the basal plane of the gold particle
and a neighboring “bare” Ti atom of the support. The energy
of co-adsorbed CO and O2 does not seem to be affected by
redox conditions. We repeated the calculation for two different cases, one in which an extra oxygen vacancy was formed
by removing O(8), and another in which an O2 molecule is
adsorbed above Ti(9). In both cases, the co-adsorption energy
of CO and O2 changed by less than 0.05 eV. Co-adsorbed CO2
and O have an energy of 3.95 eV relative to gas-phase CO
and O2. The energy of the transition state for the reaction
CO + O2 !CO2 + O is 1.19 eV relative to gas-phase reactants and yields an energy barrier of 0.40 eV. The relaxed
structures and energies associated with this CO oxidation
path are shown in Figure 2.
We then considered a second oxidation pathway, which
does not directly involve the support and is similar to that
DOI: 10.1002/ange.200461699
Angew. Chem. 2005, 117, 1858 –1860
Angewandte
Chemie
Figure 2. a, b) Relaxed geometries of the initial, transition, and final
states, and c) energy profiles for CO oxidation on a [Au10] cluster. Blue
line: [Au10] supported on TiO2(110), CO oxidation takes place at the
Au/TiO2 interface (path a); black line: [Au10] supported on TiO2(110),
CO oxidation takes place solely on the Au particle (path b); red line:
unsupported cluster with the bottom three atoms kept fixed at the
positions as they would be if the oxide were present.
proposed for a model [Au10] cluster.[8] To obtain an adsorption
energy for co-adsorbed CO and O2 on the low-coordinated
gold atoms without any direct bonding to the oxide, we
performed a calculation with O2 bonded to the bridge site
between the Au atoms 5 and 6 (Figure 1) in a geometry that is
very similar to the minimum-energy geometry described by
Lopez and Nørskov, [8] but with the extra constraint so that the
z coordinate of the O atom bonded above the bare Ti(7) is
kept within a range of 0.5 to Au(5). This configuration is
metastable, but can still contribute to the kinetics. It has an
energy of 1.15 eV relative to gas-phase CO and O2, which is
in agreement with 0.9 eV calculated for a stand-alone [Au10]
cluster.[8] The use of a constraint to get a realistic configuration is necessary, because for such a small particle it is
difficult to have a co-adsorption geometry that involves lowcoordinated Au atoms and no interaction with the support.
Co-adsorbed CO2 and O have an energy of 4.06 eV relative
to gas-phase CO and O2. The transition-state energy is
0.79 eV relative to gas-phase CO and O2 and yields an
oxidation barrier of 0.36 eV. The relaxed structures and
energies associated with this oxidation pathway of CO are
also shown in Figure 2. As can be seen from this figure, in both
oxidation pathways, the cluster changes its shape slightly to
minimize the total energy.
To gain insight into the role of the oxide in this pathway,
we repeated the same calculations, this time without the
Angew. Chem. 2005, 117, 1858 –1860
www.angewandte.de
presence of TiO2, and with the bottom three Au atoms fixed at
the positions they would have if the oxide were present. The
comparison between the unsupported and the supported
cluster is shown in Figure 2. The energy of co-adsorbed CO
and O2 is 1.12 eV, only 0.03 eV higher than the energy of the
same configuration for the supported cluster. The transitionstate energy is 0.15 eV higher for the unsupported cluster and
yields an energy barrier of 0.48 eV. There is a significant
difference in the final state (CO2 and O) energy. This
difference is an artifact related to the restructuring of the
supported cluster induced by the O atom, as can be seen in
Figure 2. The binding energy of the O atom is 0.9 eV,
relative to gas-phase O2, which is 0.6 eV lower than the
binding energy of the O atom to the unsupported cluster,
where relaxation is not fully allowed.
The conclusion from the density functional calculations is
that there are two possible reaction mechanisms for CO
oxidation on a [Au10] cluster that is supported on rutile TiO2.
The first pathway is similar to those previously reported,[14, 15]
and can only take place at a specific ensemble at the edge of
the Au/TiO2 interface. The other pathway takes place solely
on the gold particle, with the support having a small effect on
the energetics. Both pathways involve low-coordinated Au
atoms to stabilize the reactants. The two pathways show
similar activation energies for CO oxidation in the range 0.36–
0.40 eV, which is very close to the activation energy of 0.36 eV
measured by Haruta et al.,[17] close to the range of 0.15–
0.25 eV reported by Valden et al. for STM-characterized
catalysts,[18] and within the range of values between 0.16 and
0.60 eV reported by Bamwenda et al.[12] and Choudary
et al.[19]
Our findings give further support to the notion that lowcoordinated Au sites are essential to the reactivity of Au
nanoparticles, as suggested in other theoretical and experimental works.[4, 7–11] In addition, our results show why some
experiments may uncover effects due to interface sites. Most
importantly, the finding of the pure Au pathway shows that
there is at least one reaction channel open for all supports,
independent of the ability of the support to provide O2 or to
stabilize intermediates. This result explains why activities of
several different gold catalysts have been found to depend
mainly on the size of the gold particle or, equivalently, the
concentration of corner atoms on the gold particle.[10] Finally,
we note that oxygen-atom vacancies are clearly important in
stabilizing Au atoms on the surface. Without them the Au
particle hardly binds to the support.[20]
Our results suggest that the art of making Au atoms
reactive can be reduced to the question of controlling the
structure of Au particles at the nanoscale. One has to create
many interface sites or maximize the number of lowcoordinate Au atoms in the particles.
Experimental Section
The calculations were carried out by using density functional theory
(DFT); the DACAPO package was employed.[21] The ionic cores and
their interactions with valence electrons were described by ultrasoft
pseudopotentials.[22] Exchange and correlation effects were taken into
account by using the generalized gradient approximation (GGA) and
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1859
Zuschriften
the revised Perdew–Burke–Ernzerhof (RPBE) functional.[23] The
wave function was expanded in a plane-wave basis with a kineticenergy cutoff of 25 Ry. The ionic degrees of freedom were relaxed by
using a conjugate-gradient minimization until the root of the mean
squared-force component was smaller than 0.2 eV 1. Further
relaxation of selected structures resulted in insignificant changes in
the calculated energies. The rutile TiO2(110) substrate was modeled
by a two-layered slab, with a (4 2) surface unit cell. The TiO2 slabs
are separated by 15 of vacuum. First, we obtain a relaxed geometry
for the supported cluster by allowing all Au and topmost Ti and O
atoms to relax. In the calculations for adsorbed molecules on the
cluster, only the coordinates of adsorbate and gold atoms were
optimized. Three oxygen vacancies were placed on one of the
bridging oxygen rows, with the cluster on top connected by three Au
atoms of the basal plane. We used a ratio of about three Au atoms per
vacancy as a consequence of experimental STM results.[20]
To find the activation energy for the first CO oxidation pathway,
which involves the Au/TiO2 interface, we located the transition state
by decreasing the distance between the C atom of the CO molecule
and the O atom of the O2 molecule until we found a maximum in the
energy at a CO interatomic distance of 1.8 . To ensure that the
actual transition state was reached, we repeated the series of
calculations. We started this time from CO2 and stretched the CO
bond until it started to break at 1.8 , which was at a very similar
geometry to that of the reverse reaction. For the second CO oxidation
pathway, which does not directly involve support atoms, we placed
CO and O2 on the cluster in a configuration similar to that found in
the transition state reported in reference [8] and kept the CO bond
length fixed at 2.8 . All other degrees of freedom of the adsorbate
and the gold atoms were allowed to relax. We validated this
approximation in retrospect by checking that the forces on the
atoms were very small, thus verifying that the system is at a saddle
point. For both oxidation paths, we tried several initial geometries for
the system to ensure that for each instance (initial, transition, or final
state) the minimum-energy structure was found.
[13] S. Carrettin, P. Concepcion, A. Corma, J. M. L. Nieto, V. F.
Puntes, Angew. Chem. 2004, 116, 2592; Angew. Chem. Int. Ed.
2004, 43, 2538.
[14] L. M. Molina, M. D. Rasmussen, B. Hammer, J. Chem. Phys.
2004, 120, 7673.
[15] Z.-P. Liu, X.-Q. Gong, J. Kohanoff, C. Sanchez, P. Hu, Phys. Rev.
Lett. 2003, 91, 266 102.
[16] D. C. Meier, D. W. Goodman, J. Am. Chem. Soc. 2004, 126, 1892.
[17] M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M. J. Genet,
B. Delmon, J. Catal. 1993, 144, 175.
[18] M. Valden, S. Pak, X. Lai, D. W. Goodman, Catal. Lett. 1998, 56,
7.
[19] T. V. Choudary, C. Sivadinarayana, C. C. Chusuei, A. K. Darye,
J. P. Fackler, D. W. Goodman, J. Catal. 2002, 207, 247.
[20] E. Wahlstrm, N. Lopez, R. Schaub, P. Thostrup, A. Rønnau, C.
Africh, E. Lægsgaard, J. K. Nørskov, F. Besenbacher, Phys. Rev.
Lett. 2003, 90, 026 101.
[21] The DACAPO plane wave/pseudopotential DFT code is available as open-source software at http://www.fysik.dtu.dk/
CAMPOS/.
[22] D. Vanderbilt, Phys. Rev. B 1990, 41, 7892.
[23] B. Hammer, L. B. Hansen, J. K. Nørskov, Phys. Rev. B 1998, 59,
7413.
Received: August 18, 2004
Revised: October 21, 2004
Published online: February 14, 2005
.
Keywords: density functional calculations · gold ·
heterogeneous catalysis · nanostructures · titanium oxide
[1] B. Hammer, J. K. Nørskov, Nature 1995, 376, 238.
[2] M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, J. Catal. 1989,
115, 301.
[3] M. Valden, X. Lai, D. W. Goodman, Science 1998, 281, 1647.
[4] G. Mills, M. S. Gordon, H. Metiu, Chem. Phys. Lett. 2002, 359,
493.
[5] A. Sanchez, S. Abbet, U. Heiz, W.-D. Schneider, H. Hkkinen,
R. N. Barnett, U. Landman, J. Phys. Chem. A 1999, 103, 9573.
[6] Q. Fu, H. Saltsburg, M. Flytzani-Stephanopoulos, Science 2003,
301, 935.
[7] M. Mavrikakis, P. Stoltze, J. K. Nørskov, Catal. Lett. 2000, 64,
101.
[8] N. Lopez, J. K. Nørskov, J. Am. Chem. Soc. 2002, 124, 11 262.
[9] C. Lemire, R. Meyer, S. Shaikhutdinov, H.-J. Freund, Angew.
Chem. 2004, 116, 121; Angew. Chem. Int. Ed. 2004, 43, 118.
[10] N. Lopez, T. V. W. Janssens, B. S. Clausen, Y. Xu, M. Mavrikakis,
T. Bligaard, J. K. Nørskov, J. Catal. 2004, 123, 232; N. Lopez, J. K.
Nørskov, T. V. W. Janssens, A. Carlsson, A. Puig-Molina, B. S.
Clausen, J.-D. Grunwaldt, J. Catal. 2004, 125, 86.
[11] R. Meyer, C. Lemire, S. K. Shaikhutdinov, H.-J. Freund, Gold
Bull. 2004, 37, 72.
[12] G. R. Bamwenda, S. Tsubota, T. Nakamura, M. Haruta, Catal.
Lett. 1997, 44, 83.
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.de
Angew. Chem. 2005, 117, 1858 –1860
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